In a homogeneous combustion by forming a homogeneous mixture in a cylinder and by igniting and burning the homogeneous mixture at an ignition timing in the last stage of the compression stroke, if a tumbling flow is formed in the cylinder by the intake air fed into the cylinder, disturbance due to the tumbling flow is made present in the cylinder at the ignition timing by sustaining the tumbling flow up to the ignition timing in the last stage of the compression stroke, and the combustion speed of the homogeneous mixture is increased by the disturbance, then a good homogeneous combustion can be realized.
In order to sustain the tumbling flow up to the ignition timing in the last stage of the compression stroke, there have been proposed direct fuel injection-type spark ignition internal combustion engines for forming a strong tumbling flow in a cylinder by arranging an intake flow control valve in the intake port and by feeding the intake air through the intake flow control valve into the cylinder along the upper wall of the intake port (see, for example, JP-A-2005-180247, JP-A-2004-190548 and JP-A-2002-227651).
In the above direct fuel injection-type spark ignition internal combustion engines, when the intake air is to be fed through the intake flow control valve into the cylinder along the upper wall of the intake port, the intake port is throttled by the intake flow control valve. Therefore, a strong tumbling flow can be formed in the cylinder without any particular problem when the required intake air amount is relatively small. When the required intake air amount becomes relatively large, however, the intake air becomes in short supply if the intake port is throttled by the intake flow control valve. Therefore, a strong tumbling flow cannot be formed in the cylinder by using the intake flow control valve.
In the homogeneous combustion in which the air-fuel ratio of a homogeneous mixture is leaner than the stoichiometric air-fuel ratio, the intake air is required in a relatively large amount. At this time, if a strong tumbling flow cannot be formed in the cylinder, the combustion speed becomes very slow and it becomes difficult to obtain a desired engine output.
Even when the homogeneous mixture has an air-fuel ratio which is the stoichiometric air-fuel ratio or a rich air-fuel ratio, it is desirable that the combustion speed is quickened by the disturbance in the cylinder. Namely, if a strong tumbling flow can be formed in the cylinder without the need of providing the intake flow control valve, then the engine intake system may not become complex.
It is therefore an object of the present invention to provide a direct fuel injection-type spark ignition internal combustion engine which is capable of forming, in the cylinders thereof, a strong tumbling flow which is sustained up to the ignition timing when a homogeneous combustion is executed at an air-fuel ratio leaner than the stoichiometric air-fuel ratio and to provide a direct fuel injection-type spark ignition internal combustion engine which is capable of forming, in the cylinders thereof, a strong tumbling flow without the need of using intake flow control valves.